Shrub Recruitment Research Articles

Experiments on the mechanism of tree and shrub establishment in temperate grassy woodlands: Seedling emergence

Field experiments were designed to examine tree and shrub seedling emergence in temperate grassy woodlands on the New England Tablelands. The effects of study sites, intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds and ant seed theft on seedling emergence were tested in two field experiments. Six tree and seven shrub species were used in the experiments and their cumulative emergence was compared with laboratory germination studies. All species used in field experiments had lower cumulative emergence than those in laboratory germination studies despite prolonged periods of above average rainfall before and after seeds were sown. Eucalypt species emerged faster in the field than the shrub species and generally attained higher cumulative emergence than the shrubs. Spatial effects of sites and patches within sites, and of previous grazing history did not strongly influence patterns of seedling emergence in most species. Ground and litter cover generally did not enhance or suppress the emergence of seedlings, although the removal of cover in recently grazed areas enhanced the emergence of some species. Burning enhanced the emergence of some tree and shrub species where plots had more fuel and intense fires, but this effect was not strong. Compared with other treatments, seedbed manipulations produced the strongest effects. In the absence of both invertebrate and vertebrate predators, seedling emergence was lower for surface-sown seed, compared with seed sown on scarified soil surfaces. Higher seedling emergence of buried seeds in the presence of invertebrate predators probably resulted from the combined effects of predator escape and enhanced moisture status of the germination environment. Some promotion of emergence was achieved for all species in most sown treatments probably as a result of a prolonged above average rainfall. In contrast, the natural recruitment of trees and shrubs was negligible in experimental plots, highlighting the importance of seed supply and dispersal as ultimate determinants of recruitment.

Fire season effects on the recruitment of non‐sprouting serotinous Proteaceae in the eastern (bimodal rainfall) fynbos biome, South Africa

Abstract Research in Mediterranean‐climate shrublands in both South Africa and Australia shows that recruitment of proteoid shrubs (non‐sprouting, serotinous Proteaceae) is best after warm‐season (summer and autumn) fires and worst after cool‐season (winter and spring) ones. This pattern has been attributed to post‐dispersal seed attrition as well as size of pre‐dispersal seed reserves. Here we investigate patterns of post‐fire recruitment for four proteoid species in the eastern part of South Africa's fynbos biome, which has a bimodal (spring and autumn) rainfall regime. Despite the lack of significant differences in recruitment between cool‐ and warm‐season burns, we find some evidence for favourable recruitment periods following fires in spring and autumn, immediately before, and coinciding with, the bimodal rainfall peaks. This suggests that enhanced recruitment is associated with conditions of high soil moisture immediately after the fire, and that rapid germination may minimize post‐dispersal seed attrition. In two of the species, we also find a shift from peak flowering in winter and spring in the Mediterranean‐climate part of the fynbos biome, to summer and autumn flowering in the eastern part. Because these two species are only weakly serotinous, warm‐season flowering would result in maximal seed banks in spring, which could explain the spring recruitment peak, but not the autumn one. We conclude that eastern recruitment patterns differ significantly from those observed in the western and central parts of the biome, and that fire management protocols for the east, which are currently based on data and experience from the winter‐rainfall fynbos biome, need to be adjusted accordingly. Fire managers in the eastern fynbos biome should be less constrained by requirements to burn within a narrow seasonal range, and should therefore be in a better position to apply the required management burns.

The rhythm of savanna patch dynamics

1. Patch dynamics is a new, potentially unifying mechanism for the explanation of tree-grass coexistence in savannas. In this scale-explicit paradigm, savannas consist of patches in which a cyclical succession between woody and grassy dominance proceeds spatially asynchronously. The growing ecological and economic problem of shrub encroachment is a natural transient phase in this cycle. 2. An important step towards understanding patterns at the landscape scale is achieved by investigating mechanisms at a smaller scale. We developed the spatially explicit individual-based simulation model SATCHMO to test the null hypothesis that cyclical succession cannot emerge from a realistic patch scale simulation model of the population dynamics of savanna woody species. 3. We calculated the partial temporal autocorrelation coefficient for 100 simulated time series of shrub cover over 500 years for time lags of up to 200 years to establish the existence and duration of successional cycles. We found a significant positive autocorrelation indicating the existence of cycles with a typical duration of about 33 years. 4. The shrub size frequency distributions over the course of a cycle showed shifts from dominance of small shrub sizes towards larger sizes during the increasing phase of a cycle and the reverse in the declining phase. This supports the three phase explanation as follows: (i) an initial phase when spatially and temporally overlapping favourable conditions lead to mass recruitment of shrubs; (ii) a build-up phase when the shrub cohort grows; and (iii) a break-down phase when increased competition due to crowding and unfavourable conditions lead to the break-down of the shrub cohort. The frequency distribution of shrub age at death over 10 simulations was also in agreement with this explanation. 5. We investigated the relationship between shrub cover, annual precipitation and time-lagged shrub cover to identify the driver of the cyclical successions. More than 90% of the variation in shrub cover was explained by shrub cover of the previous year, precipitation, and their interaction. 6. With the demonstration of precipitation-driven cyclical succession at the patch scale, we show that the mechanistic, temporal component of patch dynamics can be used to explain tree-grass coexistence in semi-arid savannas.

Cross-Scale Patterns in Shrub Thicket Dynamics in the Virginia Barrier Complex

To interpret broad-scale erosion and accretion patterns and the expansion and contraction of shrub thickets in response to sea level rise for a coastal barrier system, we examined the fine-scale processes of shrub recruitment and mortality within the context of the influence of ocean current and sediment transport processes on variations in island size and location. We focused on Myrica cerifera shrub thickets, the dominant woody community on most barrier islands along the coastline of the southeastern USA. Observations suggest that M. cerifera, a salt-intolerant species, is increasing in cover throughout the Virginia barrier islands, yet rising sea level in response to climate change is increasing erosion and reducing island area. Our objective was to explain this apparent paradox using pattern–process relationships across a range of scales with a focus on ocean currents and sediment transport interacting with island characteristics at intermediate scales. Multi-decadal comparisons across scales showed a complex pattern. At the scale of the entire Virginia barrier complex, modest decreases in upland area were accompanied by large increases in shrub area. Responses were more variable for individual islands, reflecting inter-island variations in erosion and accretion due to differences in sediment transport via ocean currents. Several islands underwent dramatic shrub expansion. Only for within-island responses were there similarities in the pattern of change, with a lag-phase after initial shrub colonization followed by development of linear, closed canopy thickets. Understanding the fine-scale processes of shrub seedling establishment and thicket development, in conjunction with the influence of ocean currents and sediment transport, provides a framework for interpreting island accretion and erosion patterns and subsequent effects on shrub thicket expansion or contraction across scales of time and space.

Response of rock-outcrop and fringing vegetation to disturbance by fire and drought

A comparative study of vegetation responses to fire and drought investigated whether species regeneration mode, seedling density response (SDR) and seedling shoot growth rate varied significantly in relation to disturbance agent (fire and drought), habitat type (rock-outcrop and fringing vegetation) and plant growth form. A three-way ANOVA showed that SDR varied significantly in relation to all three categorical variables and most strongly in relation to disturbance agent. Seeders comprised 87% of the post-fire flora and 99.3% of the peak seedling population in rock-outcrop habitat, while resprouters were much more prominent in fringing woodland. Species SDRs and seedling growth rates were generally much higher after fire. Fire produced a high SDR with high shrub, grass and ephemeral therophyte components, and drought produced a low SDR dominated by grasses and herbs and inhibited shrub and ephemeral recruitment. Post-fire obligate-seeder shrubs behaved as facultative resprouters after drought. Some species exhibited SDRs equivalent to fire and drought, others appeared to require fire for regeneration, while others recruited more successfully after drought. This spectrum of responses indicated a range of optimal disturbance environments, depending on species, which was only partly consistent with the hypothesis that species exhibit essentially the same life-history syndromes in response to fire and drought. The dominance of seeders in outcrop vegetation appeared to be related to skeletal soil, higher disturbance frequency and soil trophic conditions, rather than low fire frequency.

Fire season and intensity affect shrub recruitment in temperate sclerophyllous woodlands

The season in which a fire occurs may regulate plant seedling recruitment because of: (1) the interaction of season and intensity of fire and the temperature requirements for seed release, germination and growth; (2) post-fire rainfall and temperature patterns affecting germination; (3) the interaction of post-fire germination conditions and competition from surrounding vegetation; and (4) the interaction of post-fire germination conditions and seed predators and/or seedling herbivores. This study examined the effects of different fire intensities and fire seasons on the emergence and survival of shrubs representing a range of fire response syndromes from a summer rainfall cool climate region. Replicated experimental burns were conducted in two seasons (spring and autumn) in 2 consecutive years and fuel loads were increased to examine the effects of fire intensity (low intensity and moderate intensity). Post-fire watering treatments partitioned the effects of seasonal temperature from soil moisture. Higher intensity fires resulted in enhanced seedling emergence for hard-seeded species but rarely influenced survival. Spring fires enhanced seedling emergence across all functional groups. Reduced autumn recruitment was related to seasonal temperature inhibiting germination rather than a lack of soil moisture or competition. In Mediterranean-type climate regions, seedling emergence has been related to post-fire rainfall and exposure of seeds to seed predators. We think a similar model may operate in temperate summer rainfall regions where cold-induced dormancy over winter exposes seeds to predators for a longer time and subsequently results in recruitment failure. Our results support the theory that the effect of fire season is more predictable where there are strong seasonal patterns in climate. In this study seasonal temperature rather than rainfall appears to be more influential.

Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River, USA

Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over-winter survival of first-year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field-observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among-species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over-winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post-colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd.

Linking a population model with an ecosystem model: Assessing the impact of land use and climate change on savanna shrub cover dynamics

In semiarid savannas of Southern Africa current land use practices and climate change may lead to substantial changes of vegetation structure in the near future, however uncertainty remains about the potential consequences and the magnitude of change. In this paper we study the impact of climate change, cattle grazing, and wood cutting on shrub cover dynamics in savannas of the southern Kalahari. We use an established savanna ecosystem model to simulate landscape dynamics in terms of rainfall, fire and distribution of the dominant tree Acacia erioloba. We then incorporate these data into a spatial population model of the common, fleshy-fruited shrub Grewia flava and investigate shrub cover dynamics for a period of 100 years. Depending on the intensity of commercial wood cutting practices tree removal of A. erioloba led to a strong decline of the G. flava population, as shrub recruitment is concentrated in tree sub-canopies due to bird-mediated seed dispersal. Under climate change shrub cover slightly decreased with decreasing precipitation and was unchanged with increase in precipitation variability. Contrarily, grazing by cattle strongly increased shrub cover and facilitated shrub encroachment because of cattle-induced distribution of G. flava seeds into the matrix vegetation. Knowledge of the latter process is particularly important because shrub invasion is a major concern for conservation and savanna rangeland management as a result of its adverse effects on livestock carrying capacity and biodiversity.

Ecological anachronisms in the recruitment of temperate light‐demanding tree species in wooded pastures

Summary Light‐demanding trees and thorny shrubs in temperate plant communities may reflect adaptations to now‐extinct large grazers, such as aurochs and tarpans, rendering these adaptations ecological anachronisms. We explored the ecological functions of plant traits of Quercus robur and Prunus spinosa in areas grazed by cattle and horses, the domesticated descendants of aurochs and tarpans. Specifically, we tested the hypothesis that grazing induces a shifting mosaic of grassland, shrub thickets and woodlands through the key process of associational resistance: the protection of palatable young trees by thorny shrubs. An exclosure experiment with transplanted Q. robur seedlings revealed that Q. robur grew best in grassland exclosures and on the edge of thorny shrub thickets, which may be viewed as an optimal balance between sufficient protection from large herbivores and sufficient light availability. A cross‐site comparison of four floodplain woodlands in north‐western Europe showed that Q. robur can regenerate in the presence of large herbivores through spatial association with P. spinosa. However, we found that expansion of P. spinosa shrubs and Q. robur coincided with periods of low rabbit abundance and not with livestock density. From this, it appears that the process of associational resistance does not work with rabbits. Synthesis and applications. With extensive grazing by large (domesticated) grazers in temperate floodplains, a shifting mosaic of grassland, shrubs and trees may develop that has high conservation value. Palatable, light‐demanding Q. robur seedlings can successfully regenerate in spiny P. spinosa shrubs through associational resistance. This process does not offer protection from abundant small herbivores, such as rabbits, that can inhibit the recruitment of shrubs and trees in this mosaic vegetation. In floodplain meadows frequent flooding may be an efficient way to reduce rabbit populations, with dry conditions in summer and wet in winter. When floodplain meadows are combined with adjacent higher grounds, large herbivores can escape the floods through migration.

FIRE EFFECTS ON PLANT DIVERSITY IN SERPENTINE VS. SANDSTONE CHAPARRAL

Fire contributes to the maintenance of species diversity in many plant communities, but few studies have compared its impacts in similar communities that vary in such attributes as soils and productivity. We compared how a wildfire affected plant diversity in chaparral vegetation on serpentine and sandstone soils. We hypothesized that because biomass and cover are lower in serpentine chaparral, space and light are less limiting, and therefore postfire increases in plant species diversity would be lower than in sandstone chaparral. In 40 pairs of burned and unburned 250-m2 plots, we measured changes in the plant community after a fire for three years. The diversity of native and exotic species increased more in response to fire in sandstone than serpentine chaparral, at both the local (plot) and regional (whole study) scales. In serpentine compared with sandstone chaparral, specialized fire-dependent species were less prevalent, mean fire severity was lower, mean time since last fire was longer, postfire shrub recruitment was lower, and regrowth of biomass was slower. Within each chaparral type, the responses of diversity to fire were positively correlated with prefire shrub cover and with a number of measures of soil fertility. Fire severity was negatively related to the postfire change in diversity in sandstone chaparral, and unimodally related to the postfire change in diversity in serpentine chaparral. Our results suggest that the effects of fire on less productive plant communities like serpentine chaparral may be less pronounced, although longer lasting, than the effects of fire on similar but more productive communities.

Experiments on the mechanism of tree and shrub establishment in temperate grassy woodlands: Seedling emergence

AbstractField experiments were designed to examine tree and shrub seedling emergence in temperate grassy woodlands on the New England Tablelands. The effects of study sites, intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds and ant seed theft on seedling emergence were tested in two field experiments. Six tree and seven shrub species were used in the experiments and their cumulative emergence was compared with laboratory germination studies. All species used in field experiments had lower cumulative emergence than those in laboratory germination studies despite prolonged periods of above average rainfall before and after seeds were sown. Eucalypt species emerged faster in the field than the shrub species and generally attained higher cumulative emergence than the shrubs. Spatial effects of sites and patches within sites, and of previous grazing history did not strongly influence patterns of seedling emergence in most species. Ground and litter cover generally did not enhance or suppress the emergence of seedlings, although the removal of cover in recently grazed areas enhanced the emergence of some species. Burning enhanced the emergence of some tree and shrub species where plots had more fuel and intense fires, but this effect was not strong. Compared with other treatments, seedbed manipulations produced the strongest effects. In the absence of both invertebrate and vertebrate predators, seedling emergence was lower for surface‐sown seed, compared with seed sown on scarified soil surfaces. Higher seedling emergence of buried seeds in the presence of invertebrate predators probably resulted from the combined effects of predator escape and enhanced moisture status of the germination environment. Some promotion of emergence was achieved for all species in most sown treatments probably as a result of a prolonged above average rainfall. In contrast, the natural recruitment of trees and shrubs was negligible in experimental plots, highlighting the importance of seed supply and dispersal as ultimate determinants of recruitment.

Lowland boreal forests characterization in Algonquin Provincial Park relative to beaver (Castor canadensis) foraging and edaphic factors

Beaver (Caster canadensis) foraging and edaphic conditions can modify the vegetational characteristics of woody plant community in lowland boreal forests. Effective management of these areas requires an understanding of the relative contribution of these factors in shaping the woody plant community structure. Our objective was to quantify the effects of herbivory by beavers and edaphic conditions on woody plant community organization of lowland boreal forests surrounding beaver ponds. Woody vegetation and soils were sampled at 15 ponds occupied by beavers and one other pond abandoned by them in southern Algonquin Park, Ontario. We measured spatial variation in plant diversity, foraging rates and sapling recruitment of trees and shrubs along gradients of beaver foraging intensity and soil moisture, P, K, Mg, and pH. Beavers fed preferentially on a small number of deciduous species and the number of cut stems declined sharply with increasing distance from ponds. Conifers increased in relative dominance to deciduous species in the presence of beavers. Plant species richness and stem and basal area diversity peaked at intermediate distances (about 25 m) from ponds. Sapling recruitment by non-preferred species was positively related to foraging intensity. Total stem abundance and basal area and sapling recruitment by four preferred species (Populus tremuloides, Acer rubrum, Acer saccharum and Corylus cornuta) were negatively related to foraging intensity. However, by including Alnus rugosa and Salix bebbiana (also preferred by beavers) these patterns changed, becoming positively related to foraging intensity. There was also a pronounced gradient in soil moisture, which also decreased with distance from ponds. The other measured edaphic variables did not vary consistently with distance from ponds. Sapling recruitment in mesic versus xeric species varied consistently with hydrid conditions along the moisture gradient, such that variation in moisture also could produce the observed pattern of plant diversity. Diversity patterns changed three years after beaver abandonment of a pond, though sapling recruitment patterns in preferred and non-preferred species around the abandoned pond were similar to the occupied ponds. These observations suggest spatial variation in woody plant richness and diversity could be determined by combined effects of both herbivory (disturbance by beavers) and variable responses of different species to edaphic conditions.

Impact of sustained drought on a semi‐arid Colophospermum mopane savanna

Vegetation change on different substrate types was measured over 15 years in semi‐arid Mopani Veld. Baseline data were collected in the terminal year (1982) of a decade of above‐average rainfall, whilst the period of monitoring was characterised by below‐average rainfall, with severe drought years occurring in 1988/1989 and 1991/1992. Grazing pressure was, on average, alleviated over the period of recording so that changes reflect primarily the effect of rainfall. Long‐lived perennial grasses had declined markedly, annual and micro‐perennial grasses had increased markedly, and the proportion of forbs had increased. Despite 7–17 years since the replacement of livestock grazing with wildlife at lower stocking rates, there was no evidence of an improvement in sward or soil condition, owing ostensibly to the effect of drought. The extent of change in botanical composition and in sward condition was influenced by substrate type. The rate of growth of the woody component had been slow: the average per annum increase in the radius of trees was 0.67 mm. For the dominant species, Colophospermum mopane, only individuals of less than 1 m in height consistently showed an increase in size. The basal area of dead standing trees had increased threefold. Complete mortality or substantial topkill of shrub species, in particular of Grewia spp., Ximenia americana, and Combretum apiculatum, and of adults of the latter, had occurred owing to drought. Shrub mortality was most pronounced on colluvial soils and sandveld. All species exhibited partial dieback of the crown. Overall, shrub recruitment has been limited except for that of H. mopane on colluvial soils but not on other substrate types, which is therefore likely to increase further its dominance on this substrate at the expense of species richness. Grazing pressure on desirable perennial grasses and browsing pressure on palatable woody plants has probably increased because of their decline, even if animal abundance has not increased, which will have a negative effect on their re‐establishment. Acting in opposition, dieback of trees and shrubs has probably created a window of opportunity for the recovery of perennial grasses, but it would likely require an extended run of above‐average years to reverse the changes in herbaceous vegetation, for which the availability of seed of perennial grasses is likely to be a major constraint.

Runoff and micromorphological properties of a grazed haplargid, near Cobar, NSW, Australia

We investigated the effects of 2 different grazing regimes on the surface soil properties of a dunefield land system in the semi-arid woodlands of eastern Australia. Sandy siliceous, thermic Xeric Haplargids (Siliceous Sands, Uc1·23) occur on the sandy, 2–4-m-high longitudinal dunes. Fine-loamy, siliceous, thermic Xeric Haplargids (Massive Red Earths, Uc2·13) occur in the swales between the dunes. We compared very high-intensity grazing (approx. 1 year) by feral goats with low-intensity grazing (approx. 4 years) by sheep. A rainfall simulator, applying water at the rate of 30 mm/h, measured the hydraulic properties of the surface soils formed under the 2 different grazing regimes. We examined undisturbed samples of the upper 5-cm layer of the soil surface using micromorphological techniques. In the swales, there were no differences in the effects of the 2 grazing regimes on soil properties. At low-intensity sheep grazing (0·2–0·3 sheep/ha), the soil surface on the dunes remained in an excellent condition. The surface had a good vegetative cover and consisted of either loosely packed sand grains, or areas where the sand grains were bonded together by clay and organic matter to form an organic crust. The total carbon content of the 0–2 cm depth of soil was 0·86%. Both soil surfaces have a high infiltration rate (i.e. >30 mm/h) and also appear to contain stable microaggregates of parna material distributed among the eolian sand grains. Very high-intensity goat grazing (up to 4·0 goats/ha) rapidly depleted the perennial grasses, killed most of the shrubs, and converted the soil surface on the dunes to one highly susceptible to erosion by wind. The low total carbon content (depth 0–2 cm) of 0·3% and absence of iron-stained clay coatings on the sands further support this view. The surface soil on the dunes in the very high-intensity goat-grazing plots consisted of either loosely packed sand grains or areas where poorly orientated clays coated the sand grains to form a strong, physical crust. We suggest that the physical crust may cause a change in the hydrology of the land system which may enhance the conditions for recruitment of unpalatable shrubs in the dune–swale interface. This increase in unpalatable shrubs and decrease in perennial grasses caused by the very high intensity goat grazing is therefore detrimental to the long-term productivity of these semi-arid lands.

Equilibrium response of riparian vegetation to flow regulation in the Platte river, Nebraska

The Platte River in central Nebraska responded to water development by rapid channel narrowing and expansion of native riparian woodland. Woodland expanded most rapidly in the 1930s and 1950s; open channel and woodland area stabilized in the 1960s and have remained stable for most reaches into the mid-1990s, despite relatively low flows and infrequent peak flows in the past decade. Open channel area may have been maintained or increased under recent lower flows because of increased erodibility of the floodplain as it has aggraded, developed vertical banks and as its woodland vegetation has become older, sparser and less protective of banks. One section of the Platte River, near Grand Island, has disequilibrated in the past decade by undergoing a 10% loss of channel area. The reach occurs below an area where vegetation has been removed to increase open channel area for migrating whooping and sandhill cranes and other water birds. Vegetation clearing may have liberated excess sediment, locally aggraded the channel and stimulated tree and shrub recruitment. This management practice needs to be examined before it is used more widely in the Platte River. © 1997 John Wiley & Sons, Ltd.

Equilibrium response of riparian vegetation to flow regulation in the Platte river, Nebraska

The Platte River in central Nebraska responded to water development by rapid channel narrowing and expansion of native riparian woodland. Woodland expanded most rapidly in the 1930s and 1950s; open channel and woodland area stabilized in the 1960s and have remained stable for most reaches into the mid-1990s, despite relatively low flows and infrequent peak flows in the past decade. Open channel area may have been maintained or increased under recent lower flows because of increased erodibility of the floodplain as it has aggraded, developed vertical banks and as its woodland vegetation has become older, sparser and less protective of banks. One section of the Platte River, near Grand Island, has disequilibrated in the past decade by undergoing a 10% loss of channel area. The reach occurs below an area where vegetation has been removed to increase open channel area for migrating whooping and sandhill cranes and other water birds. Vegetation clearing may have liberated excess sediment, locally aggraded the channel and stimulated tree and shrub recruitment. This management practice needs to be examined before it is used more widely in the Platte River. © 1997 John Wiley & Sons, Ltd.

Relative Effects of Herbivory by Sheep, Rabbits, Goats and Kangaroos on Recruitment and Regeneration of Shrubs and Trees in Eastern South Australia

Short-term recruitment and long-term regeneration of 18 species of shrubs and trees in over 600 populations in eastern South Australia were determined by surveying membership in life history classes. Sites were stratified using current herbivory levels by sheep, rabbits, goats and kangaroos, past herbivory by sheep, and edaphic (soil) variables. Ten species showed significant negative effects of present or past herbivory, either on recruitment, regeneration or both. Sheep grazing was the most important variable and the other herbivores frequently did not have independent effects on regeneration. There was a poor relationship within a plant species between suppression of recruitment and suppression of regeneration by herbivores. The negative effects of herbivory on recruited juveniles must exceed natural thinning before overall regeneration is affected. Some species showed major disruption of regeneration at sites with a history of heavy grazing during the 19th century regardless of their present responses to grazing. This evidence supports non-equilibrium models of vegetation dynamics and indicates the importance of taking historical and other non-equilibrium effects into account when designing studies of plant populations. If many plant species are to avoid local extinction, it will not be sufficient to rely on controlling goats and rabbits as a conservation measure while ignoring the overriding effects of sheep grazing. The pattern of grazing use of the area will have to be modified to secure long-term conservation. A regional network of small and large reserves is suggested.

Landscape and Microsite Influences on Shrub Recruitment in a Disturbed Semi-Arid Quercus-Juniperus Woodland

Seedling demography of the leguminous shrubs Leucaena pulverulenta and Acacia berlandieri was monitored to determine the functional role of microsites in the restoration of disturbed semiarid woodlands. Seeds were sown on replicated landscapes during 1989 and again in 1990 into four microsites after brush clearing: 1) Rocky, with shallow soils ( 80%), intermediate on Rocky (65%), and lowest on Herbaceous (36%) microsites. Desiccation appeared to be the primary cause of first-season mortality. Differences between the microsites in seedling height and number of leaves were significant the first planting but not the second. Overwinter mortality of Leucaena and Acacia seedlings was lowest on Cleared microsites (55 and 7%, respectively) and greatest on microsites dominated by Herbaceous vegetation (100 and 87%, respectively). Survival of established seedlings during the second growing season exceeded 80% for both species on all microsites. A. belandieri had the highest establishment potential and the greatest ecological amplitude, with recruitment after two years being comparable on Cleared, Rocky and Tree microsites (72-78 seedlings). L. pulverulenta showed marked preferences for Cleared microsites (47 seedlings established) relative to Rocky (13 seedlings), Tree (12 seedlings) and Herbaceous microsites (0 seedlings). Spatial variability in emergence between replicated landscapes (range 35 to 78%) was greater than the variability between microsites within a landscape (48 to 62%). Differences in seed disappearance associated with surface runoff from high intensity storms and losses to granivory on Tree microsites were the primary causes of differences in recruitment between landscapes. Inter-landscape variability in granivory and precipitation runoff therefore overshadowed the effects of within-landscape seed placement among first year seedlings. Generalizations of seedling safe site characteristics based solely on short-term (one year), pseudoreplicated, within-landscape studies may therefore be misleading. Failure to account for variability in important processes at greater spatial scales may significantly influence the robustness of experimental microsite study results derived from small-scale research.

Effects of habitat and growing season fires on resprouting of shrubs in longleaf pine savannas

The effects of habitat and timing of growing season fires on resprouting of shrubs were studied in second-growth longleaf pine savannas of the west Gulf coastal plain in the southeastern United States. Within the headwaters of three different drainages of the Calcasieu River in the Kisatchie National Forest in western Louisiana, replicated permanent transects were established that extended from xeric upland longleaf pine savannas into downslope hydric seepage savannas. All shrubs were mapped and tagged, and numbers of stems were counted prior to any fires. Replicated prescribed fires were set early (June) and late (August) during the 1990 growing season; maximum fire temperatures were measured within both upland and seepage habitats within each transect. Shrubs were relocated; stems were recensused two and twelve months after the fires. At least some shrubs of all species resprouted from underground organs; none regenerated solely from seed banks in the soil. There was no reduction in total numbers of stems one year after fires compared to before fires, either in the upland or in seepage savannas. In addition, there was no reduction in total numbers of stems one year after early or late growing season fires. Fire-related mortality was restricted to small shrubs (< 18 stems) and was not associated with high fire temperatures. The rate of resprouting varied among species and between habitats. Resprouting occurred more rapidly in seepage than upland savannas, but more resprouts were produced in upland than seepage savannas one year after fires. In contrast to other upland species, Vaccinium arboreum and V. elliottii delayed resprouting more than two months following fire. Stems of Rhus copallina and Pyrus arbutifolia, species with long rhizomes, increased more after fires in June than fires in August. We suggest that growing season fires may block further recruitment of shrubs into longleaf pine savannas, but reduction in numbers of large shrubs may require additional management.

Growth, flowering and recruitment of shrubs in grazed and in protected rangeland in the arid Karoo, South Africa

A three-year study involving 1300 marked shrubs of three species differing in palatability to sheep provided evidence that changes in the demographic structure of shrub populations in arid rangelands are a function of seed availability. Sheep reduced canopy size and flowering of their preferred forage species but had no effect on survivorship of seedlings or established plants during the study. Although annual turnover in all populations was low (<6%), abundant recent recruitment of non-forage species was observed on degraded rangeland. Natality of all three species was positively correlated with flower abundance. Mortality was correlated with natality because seedlings had a low probability of surviving their first year. Recruitment of both forage and non-forage species, appeared to be inhibited by competition and seedlings seldom survived near adult plants of the same growth form.